For various cutting purposes, agricultural harvesting machinery, especially choppers, utilized blades and knives which on the nonbevelled flank, are coated with a hard facing material. The hard facing material can be applied by a thermal spraying process in two stages. In a first stage, the coating is deposited by a flame spraying, plasma spraying or a like technique whereas the bonding is improved by a second stage thermal treatment which involves a fusion or sintering.
The layer can be composed of a metal matrix and a hard material, especially tungsten carbide, which is incorporated in the matrix.
The HV value for the carbide is usually 2 to 3 times greater than the corresponding value for the matrix and the hardness value for the matrix can be approximately equal to that of the underlying material or the substrate, for example, steel.
Because of the hardness differences between the hard facing layer and the substrate material, the blade suffers from a difference in wear, with the softer substrate material wearing more rapidly than the hard facing. At the cutting edge, a portion of the hard facing material may project beyond the substrate material so that the blade has a self sharpening effect in use.
Harvesting machines of the type described have been required to have greater productivity in recent years and the wear resistance of previous hard facing materials have not met the newly applied standards. Under high productivity conditions, the wear of the hard facing layer is more pronounced and the hard facing material may not project beyond the substrate material so that, rather than a self sharpening effect, there may be a rounding of the cutting edge. It, therefore, is essential to increase the resistance to wear of the hard facing material.